Existing prior art inflated planing-type or semi-planing seals on surface effect ships are generally designed to maintain a constant overpressure relative to the air cushion pressure which they contain. The amount of overpressure may be adjusted manually by use of valves which vary the effective cross-sectional area of orifices in the discharge ducts which may vent the seal air into the main air cushion. One major disadvantage of this prior art arrangement is that the seals cannot produce static stabilizing forces and moments to compensate for longitudinal and lateral shifts in weights, or changes in overall weight and weights distribution of the craft. In addition, there is no provision for direct dynamic stabilization in waves. For example, the bow seal cannot produce a force proportional to the instantaneous clearance of the bow above the water surface that would tend to lift the bow of the vehicle over waves being encountered. In the static situation, each of the prior art seals only produce a constant force while in contact with the water.
Further, existing vehicles, with rigid sidewalls, rely primarily on the hydrostatic and hydrodynamic forces acting on the sidewalls to provide for roll and pitch stability. Consequently, the designer of rigid sidewall vehicles (captured air bubble vehicles, for example) has little or no means of control over the dynamic or static characteristics of the craft in roll and pitch. Therefore, existing craft have inadequate damping characteristics in pitch and roll, which result in resonance and excessive motion in waves. This deficiency in dynamic performance is corrected by means of a properly designed device of this invention to be described hereinafter.
Prior art vehicles obtain some stabilization dynamically from the transient overpressure in the seals caused by rapid changes in seal forces which are in turn produced by seal contact with the changing water surface heights. Again, the designer formerly had little control over this type of dynamic effect, because the fixed orifice area discharge in the ducts is pre-determined by the designed-in static overpressure in the seals.
For example the prior art has no concern with the direct control of the vehicle's height above the supporting surface. All appear to control either heave, pitch, or roll, or a combination thereof, by means of sensing pressures in various components of a fully-skirted craft. The pressure in a given compartment is related only indirectly to the relative height of that compartment over the supporting surface. The varying pressures are used to activate various control means that may adjust the flow and/or volume of the compartments.
In summary, the overall performance of an air-cushion-borne vehicle relates to the suspension system (i.e., the seal, the air cushion, and the air feed system). This performance can be characterized by the various stiffnesses damping characteristics, and natural frequencies associated with air cushion vehicles. The stiffnesses, which characterize static performance, are expressed in terms of the ratio of incremental loading forces to the resulting incremental local clearances of the vehicle above the surface. The damping and natural frequency, in addition to stiffnesses, characterize the dynamic behavior of the vehicle.
Therefore it is important to distinguish between static and dynamic response, and overall stability. Static stability is characterized solely by stiffnesses of various components of the suspension system, which compensate for the vehicle's overall weight and trim. Dynamic stability performance is governed by the stiffnesses, damping characteristics, and natural frequencies of the vehicle in pitch, roll, and heave, as it encounters irregular surfaces including wave crests and troughs.